The industrial Kraft process takes wood and converts it into wood pulp for many uses. In general, the process involves cooking the wood chips in chemicals, mainly comprising a mix of sodium hydroxide and sodium sulfide, known commonly as white liquor in the pulping industry. After the cooking process, the cooked wood is separated from the liquids, the resulting liquid is commonly called black liquor, with varying chemical composition depending upon the type of wood. The black liquor is converted back into white liquor in a process commonly known as a caustic-recovery process, or chemical recovery process.
The first step in the conventional caustic recovery process is concentration of black liquor from the pulping process. Concentrated black liquor is sent to a recovery boiler to (1) recover the key chemical compounds, such as sodium carbonate, sulfides etc.; (2) combust the organics material in the black liquor; and (3) to recover energy which may be used throughout the pulp and paper mill or exported out of the plant. The smelt from the recovery boiler is generally mixed with a solution commonly known as weak liquor from the caustic recovery process, the resulting mixture of the weak liquor and the boiler smelt is commonly referred to as green liquor and generally contains sodium carbonate, sodium hydroxide, sodium hydrosulfide and may contain other compounds such as sodium sulfite, sodium thiosulfate and other process or non-process impurities.
The green liquor and calcium oxide, CaO, solids (commonly known as quicklime or burnt lime) from the downstream calciner are fed into a stirred tank reactor, generally known as slaker or lime-slaker. Two reactions, reaction (1) and reaction (2), take place in the slaker. First CaO reacts with water in the green liquor to form calcium hydroxide (Ca(OH)2, commonly known as slaked lime, hydrated lime, builders' lime, pickling lime, or Chuna) via reaction (1).CaO(s)+H2O(aq)→Ca(OH)2(s)  (1)As soon as any calcium hydroxide Ca(OH)2 is formed it begins reacting with sodium carbonate in the green liquor to form solid calcium carbonate (CaCO3), via reaction (2).Ca(OH)2(s)+Na2CO3(aq)⇄CaCO3(s)+2NaOH(aq)  (2)Reaction (1) and Reaction (2) are generally known as the slaking reaction and the causticization reaction, respectively; and both reactions occur simultaneously anytime water containing carbonate is mixed with quicklime.
The bulk of the causticization reaction takes place in the slaker. Generally, the contents from the slaker are fed into a series of stirred tanks, typically referred to as causticizers, where the reactions are allowed to proceed to near completion. The resulting solution is referred to as unclarified white liquor and contains, among other chemicals, suspended CaCO3 particles, called lime mud which are around 15 μm in size.
Thereafter, generally the lime mud is first removed from the white liquor via clarifying tanks or pressurized filters. Typical filtration equipment for this step includes pressurized tubular filters or pressurized disc filters. The resulting clarified white liquor is returned to the pulping process to cook more wood chips, and the lime mud is sent for further washing and filtration before being calcined. Calcination is the term for converting lime mud (CaCO3) into quicklime (CaO):CaCO3(s)→CaO(s)+CO2(g)  (3)
The resulting off-gas is typically cleaned and discharged to atmosphere while the produced CaO is sent back to the slaker for reaction with the green liquor.
The lime mud in the conventional caustic recovery process fouls both the calciner and any downstream gas processing equipment. Rotary kilns have been shown to tolerate the fouling caused by the lime mud and operate continuously requiring only minor shutdowns for cleaning and maintenance. As a result, the rotary kiln is the most common type of calciner applied today to calcine lime mud and the hot off-gases from the kiln are commonly used to dry incoming lime mud as they will foul any other type of equipment. The rotary kiln is a large, expensive, difficult to operate piece of equipment and the off-gases are vented to the atmosphere still containing a large quantity of high grade heat resulting in an overall thermal efficiency of around 40%. Many of the challenges in the calcination section of the conventional caustic recovery process are a direct result of the fine particle size of lime mud and its tendency to foul high temperature solids processing equipment.